It's pretty logical to make people provide PoC, I'm not going to go bug hunting because someone says "I have an exploit" chances are they probably have the code they used to test the exploit so I don't see the problem in showing an example. I'd most likely want PoC code only, pretty obvious it's an exploit if it causes the core to behave other than intended.
So if I say, "the code at this address is vulnerable because whatever, see?" And they say, "no thanks, code an exploit first" (actually, they just don't respond). What do I do? The obvious thing is to fully disclose the vulnerability and let somebody else develop an exploit for whatever purpose.
The important thing to remember is that, when this happens in an open-source program, the users can protect themselves. In a closed-source program, users are screwed until the person who puts it out fixes it. What if they're on vacation? What if they're dead? Your program is now useless.
True however if there were absolutely none then there would be no need for worrying. Since they DO exist I'm not going to assume they are good. I'll only think someone is good when they show me they are.
I don't wear a bullet proof vest while I'm walking downtown, but I know that people get shot.
I'm pretty sure that you think I'm good, but I haven't proven it.
You have to draw a line in the sand between trust and suspicion.